A bent glass sheet is commonly used for automobiles and buildings. Especially, in the field of window glass sheets for automobiles, the bent glass sheet is in great demand because of the advantages in the design of an automobile, aerodynamic characteristics or the like. A lass sheet that is mass-produced primarily by a float glass process has a flat shape at first. The flat glass sheet is formed into a bent glass sheet by bending the flat glass sheet in a secondary process by various industrial techniques. In the process of forming a bent glass sheet involving heating, the bent glass sheet tends to be quenched for tempering after or while bending.
A method for forming a bent glass sheet that is commonly used is to press a heated glass sheet between a pair of dies. For example, a glass sheet is conveyed from a heating furnace to a press molding device while being suspended from tongs (a load handling device). In connection with this method, in order to avoid generating tong marks on the surfaces of the glass sheet, it is common that a heated glass sheet is conveyed to the forming device horizontally by rolls or the like. In general, rolls are used for conveying the glass sheet horizontally but a belt is used in some cases.
For example, Japanese Laid-Open Patent Publication (Tokkai Hei) No. 3-50132 (JP A 3-50312) discloses a method in which a glass sheet retrieved from a heating furnace is conveyed by a belt to a bending section In the bending section, the heated glass sheet is pressed between upper and lower press dies together with the belt. This method is performed by an apparatus as shown in FIG. 14. In this method, a glass sheet 104 retrieved from a heating furnace 101 is conveyed to a bending section 102 in the horizontal direction. The glass sheet 104 is conveyed to a position between press dies 106a, 106b by a belt 105 operated by rolls 107, followed by being pressed by the dies 106a, 106b together with the belt 105 at the pressing position. The glass sheet is quenched for tempering at the same time. After pressing, a bent glass sheet 104' is conveyed in the horizontal direction. This method utilizes the flexibility of the belt, in order to abbreviate the reduction in the temperature of the glass sheet from heating to quenching in the case where the glass sheet is quenched for tempering.
Furthermore, an improved method for bending a glass sheet with tempering is disclosed in Japanese Laid-Open Patent Publication No. 6-40732 (JP A 6-40732). In this method, in order to realize a curved shape with high accuracy an expandable casing having a flexible thin film is used as the lower press die.
In such methods for forming a bent glass sheet involving such press molding, the glass sheet stays in the press dies during molding, regardless of what is used for conveying the glass sheet.
In view of the efficiency in producing bent glass sheets successively, methods in which bending is performed without stopping the glass sheet in the production line have been proposed and are in practical use. Methods of this type include a method in which a glass sheet is heated and softened while being horizontally conveyed in the heating furnace, and the glass sheet is allowed to gradually conform with the shape of the surface of a member for conveying the glass sheet such as an air bed. This method is efficient in the case where bent glass sheets having the same curvature are produced successively, so that this method is widely employed with various modifications (e.g., Japanese Laid-Open Patent Publication No. 7-237928 (JP A 7-237928)).
Furthermore, the following method was proposed. A curvature is provided for a conveying path arranged adjacent to an outlet of a heating furnace, and a heated glass sheet is being curved while conveyed along the conveying path by rolls (Japanese Laid-Open Patent Publication No. 55-75930(JP A 55-75930)). This method has advantages of an excellent efficiency in heating the glass sheet and enabling types (shapes) of the bent glass sheet to be changed easily compared with the method for forming a bent glass sheet utilizing the self-weight of the glass sheet as described above.
However, according to the method disclosed in JP A 55-75930, a heated lass sheet is being curved intermittently by rolls positioned on the both sides of the path for conveying the glass sheet, so that the marks of the rolls are likely to be left on the surface of the glass sheet. This disadvantageously deteriorates the quality of the bent glass sheet. The flaws and roughness on the surface of the glass sheet caused by the rolls result in optical defects that cannot be ignored, especially in the field of window glass sheets for vehicles, which is the largest use for bent glass sheets. Furthermore, in the method for bending intermittently by rolls, the degree of freedom in forming a bent glass sheet is inherently limited, so that the formable shapes are limited to a narrow range.